Cardiac cycle: Wiggers diagram
ECG, pressure, and volume
A cycle of cardiac activity (i.e. one heartbeat) can be divided into two basic phases: systole (contraction) and diastole (relaxation).
These two phases can be further subdivided into seven phases, indicated in the figure with numers:
- Phase 1: atrial contraction
- Phase 2: isovolumetric contraction
- Phase 3: rapid ejection
- Phase 4: reduced ejection
- Phase 5: isovolumetric relaxation
- Phase 6: rapid LV Filling
- Phase 7: reduced filling
This figure, known as a Wiggers diagram, shows the changes in electrical activity, pressures, and volumes that occur in the course of one heartbeat.
The cycle begins with the P wave in the ECG, which reflects the depolarization of the atria. Atrial contraction (A) pumps blood into the ventricles (B). The atrial systole lasts about 100 msec. At the end of atrial systole, the ventricles are maximally filled, a state known as the end-diastolic volumeof the left ventricle (C).
The QRS complex shows the depolarization of the ventricles. The ventricular systole takes about 270 msec. During the first stage of the ventricular systole, the myocardium contracts, causing a pressure increase in the ventricle without a volume change because the valves are still closed (isovolumetric contraction). When the pressure exceeds that in the aorta, the valves open (D).
During ventricular systole, the left ventricle ejects about 80 ml of blood: the stroke volume. The maximum pressure of 120 mm Hg is reached around mid-systole (E). At the end of the ventricular systole, pressure drops quickly (F). Once the blood in the aorta begins to flow back to the left ventricle, the semilunar valves close. This leads to a short increase in pressure in the aorta as the elastic walls recoil. This is called the dicrotic notch. At this point, the end-systolic volume is reached in the leventricle.
The T wave represents the repolarization of the ventricles. Ventricular diastole lasts till until the end of the cycle (430 msec). At (G), all valves are closed: the pressure in the ventricles is still too high for blood flow into the ventricles. This period is called isovolumetric relaxation. When the pressure in the left ventricle drops below the left atrial pressure, the valves open and blood flows into the ventricles (H). This passive filling of the ventricles occurs during the remainder of the cycle, when both atrium and ventricle are in diastole.
During the cardiac cycle, the amount of blood remaining in the ventricle when the aortic valve closes is the
During diastole, a chamber of the heart
The QRS complex of the ECG appears as the